This invention relates to ellipsometry.
One class of ellipsometers includes a light source, a polarizer, a modulator, an analyzer and at least one intensity detector. In this class of ellipsometer, the light is modulated and sensed by a photocell. The resulting electric signal is used to calculate psi and delta, which can be done by a number of known techniques including a Fourier expansion of the resulting signal.
In this class of ellipsometry, errors occur, such as for example, because of Changes in angular rotational alignment or tilt of the modulator or polarizers or sample about the optical axis or changes in the environment that cause signals to be processed in a nonlinear portion of the components of the instrument. Moreover in one type of ellipsometer in this class of ellipsometers, called a large modulation ellipsometer, the modulator varies the intensity of the beam over a defined range sufficiently large to give an easily measured signal to avoid poor signal to noise ratios.
Because of changes in the environment, corrections may need to be made such as in the software to make the necessary corrections or adjustment of voltages or adjustment of amplification level of amplifiers or sensors or attenuation levels of signals from photocells or changes in the angle of the diffraction grating if one is included or in the polarizer. For example, the photocell output signal in some ranges changes nonlinearly with respect to changes in the intensity of light and the light reflected from diffration gratings, if any are included, change nonlinearly with respect to changes in the phase. The changes in ranges of intensity and polarization state are caused by changes in reflected or transmitted light as the environment of measurement changes.
In a prior art type of ellipsometer in this class, these errors are corrected in a manner that requires time consuming calibration for different environments. The calibration of alignment is generally obtained by measuring the psi and delta of a known environment similar to the one being tested and calibrating the instrument in that known environment to provide correction factors. Only the first order and second order coefficients of the Fourier series are used in the prior art calculate delta and psi because these coefficients are known to contain sufficient information by themselves to calculate delta and psi.
This prior art type of ellipsometry has a disadvantage in that a large amount of time is necessary to correct the system by calibration and the instrument is limited in the benefits it can provide for some uses such as those in which the environment changes frequently. This disadvantage of prior art type of ellipsometers is especially severe when continuous measurements of time varying quantities are made because of the need to calibrate for different surfaces.